Myocardial Ischemia: SPECT-CT Low Dose Validation

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Case Summary

A 79-year-old white male presented with a three
day history of intermittent chest pain and dyspnea. He had a prior
history of anterior myocardial infarction and bypass surgery ten years
previously. Treadmill stress was performed as part of the imaging study.
The patient exercised to 90% of maximum predicted heart rate and
achieved 10 METS. There were no significant stress-induced ECG changes.

Imaging Findings

Rest
and stress myocardial perfusion images were acquired at the appropriate
time intervals after intravenous radiopharmaceutical administration (10
mCi at rest and 30 mCi at peak stress). Using concurrent imaging, an
exclusive technology that enables the acquisition of up to 15 datasets
into one acquisition step, gives physicians information more quickly and
increases diagnostic confidence. Acquiring multiple isotopes
simultaneously has the potential to improve timing resolution, providing
increased efficiency and productivity as well as better patient
comfort. Emission tomographic SPECT images were acquired both in 64×64
and 128×128 matrices. Figure 1 is 64×64 data with filtered
backprojection reconstruction. Figure 2 is half the 128×128 data
reconstructed with iterative reconstruction and low-dose CT attenuation
correction (Philips Astonish). The 128×128 series more accurately
depicted the extent and severity of ischemia as well as the presence of
previous anterior infarction (not seen in 64×64). Coronary angiography
subsequently found stenosis of the previous LAD graft and a stent was
deployed with good restoration of blood flow. Corresponding ECG-gated
images showed anterior hypokinesis with LVEF 42%.

Myocardial
perfusion imaging at rest and stress is a mainstay of noninvasive
diagnosis to detect coronary artery stenosis requiring intervention.
Both for initial diagnosis and subsequent risk stratification, SPECT has
proven to add incremental information to aid in patient management. The
addition of ECG-gated methods allows further additional assessment of
ventricular pump function for correlation with the regional perfusion
information. As with other techniques in both PET and SPECT, a low dose
CT acquired for calculation of attenuation correction can improve the
quality and accuracy of the emission data.1

The study
above demonstrates the improved accuracy of 128×128 myocardial perfusion
SPECT-CT utilizing half the usual data. This can allow most clinics to
decrease the administered doses of radiopharmaceutical by at least fifty
percent, significantly lowering the patient radiation burden even with
the addition of the small exposure from the low dose CT used for
attenuation correction.2 Current reconstruction methodology
and attenuation correction compensates for the lower count acquisitions.
Alternatively, there is the option for longer acquisition times for
higher counts, especially in large patients.

Further reduction in
exposure can be achieved in selected patients with stress-only imaging
where rest images are acquired only if the stress is abnormal.3